1. Field of the Invention
The present invention relates to immersion lithography, more particularly, to providing systems and methods for correcting thermally-induced aberrations due to immersion liquid heating.
2. Related Art
Lithography is a process used to create features on the surface of substrates. Such substrates can include those used in the manufacture of flat panel displays (e.g., liquid crystal displays), semiconductor wafers, circuit boards, various integrated circuits, print heads, macro/nano-fluidic substrates, and the like. During lithography, a substrate, which is disposed on a substrate stage, is exposed to an image projected onto the surface of the substrate by exposure and projection optical systems located within a lithography apparatus.
The projected image produces changes in the characteristics of a layer, for example, photoresist, deposited on the surface of the substrate. These changes correspond to the features projected onto the substrate during exposure. Subsequent to exposure, the layer can be etched or otherwise processed (e.g., developed in case of the layer being a photoresist layer) to produce a patterned layer. The pattern corresponds to those features projected onto the substrate during exposure. The patterned layer is then used to remove or further process exposed portions of underlying structural layers within or on the substrate, such as conductive, semiconductive, or insulative layers. This process is repeated, together with other steps, until the desired features have been formed on the surface, or in various layers, of the substrate. In the field of immersion lithography the exposure operation is conducted with an immersion liquid, which is typically water, between the last lens element of the projection optics and the substrate. This requires certain design modifications to the apparatus.
Lithographic systems have adjustable optics which allow for the correction of optical wave aberrations of, for example, the projection optics. Adjustments are made for lens heating effects which occur even though the lenses of a projection optics are temperature controlled. The aberration levels are measured and adjusted using both on-system and off-system metering devices such as interferometer based sensors.
Current aberration control systems only deal with slowly changing aberration effects related to lens heating. For example, U.S. Pat. No. 6,563,564, entitled Method of Operating an Optical Imaging Syste, Lithographic Projection Apparatus, Device Manufacturing method and Device Manufactured Thereby, issued to de Mol et al. on May 13, 2003 relates to lens heating and does not address the heating effect of an immersion fluid. The heating effect on the immersion fluid is instantaneous and dependant upon the delivered dose. As soon as the exposure energy is turned off the heated block of fluid is swept away by the fluid flow through an immersion lithography showerhead. The flowing fluid induces a non-symmetric temperature distribution across the width of the exposure slit which induces aberrations in the image field. The aberrations, such as defocus, change across the width of the field. This kind of instantaneous, non-symmetrical aberration effect is not corrected in current aberration control systems.
What are needed are aberration control systems and methods that effectively address instantaneous, non-symmetrical aberration effects that occur within an immersion lithography system.